Acrylic acid can be obtained by obtaining a gas comprising acrylic acid which is obtained by catalytic vapor-phase oxidation of propylene or the like, collecting the gas as an acrylic acid aqueous solution by a collection column, separating moisture and low-boiling fractions by distillation, and rectifying using a fractionating column, as a distillate.
Usually, at all of the steps after the step of collecting, a polymerization inhibitor is added for preventing polymerization of acrylic acid. Polymerization inhibitors generally used include: p-hydroquinone, p-methoxyphenol, phenothiazine, copper compounds such as alkyl copper carbamates and manganese compounds. Each of them has a boiling point higher than that of acrylic acid and is concentrated in the bottoms of the fractionating column.
The polymerization inhibitor is usually discarded together with a residual compound having a high boiling point after recovery of the acrylic acid oligomer in the bottoms by thermal decomposition. In recent years, however, several methods have been proposed for recovering and recycling a polymerization inhibitor in the bottoms of the fractionating column from the point of costs for the production of acrylic acid.
The conventional methods of recovering the polymerization inhibitors are, for example, as follows.
(1) Simple-recycling: The bottoms of a fractionating column, which contains a polymerization inhibitor and is generated in the process for producing an acrylate, is sent without modification to the step of purifying (JP 2001-181233 A).
(2) Distillation with evaporator: The bottoms of a fractionating column, which contains a polymerization inhibitor, is introduced into an evaporator and a fraction obtained by evaporation of the polymerization inhibitor using the evaporator is recycled in the step of purifying (JP 56-17331 B).
(3) Water extraction: The polymerization inhibitor is recovered from the fraction obtained in (2) by water extraction (JP 60-59889 B).
In those conventional methods of recovering polymerization inhibitors, no compounds other than polymerization inhibitors have been taken into consideration and thus there is a problem of contamination with impurities.
When the bottoms of the fractionating column or the bottoms of a thermal decomposer after thermal decomposition for decomposing the acrylic acid oligomer of the bottoms of the fractionating column is sent to the step of purifying, all impurities in the bottoms are circulated through the step of purifying. Therefore, the circulation is unrealistic because of an increase in the concentrations of impurities in the product.
Also, in the case of evaporating the polymerization inhibitor using an evaporator, an increase in load of the step of purifying at the time of circulation through the step of purifying or an increase in the concentrations of impurities in the product is brought because the most part of impurities having boiling points lower than that of a polymerization inhibitor are collected.
Also, in the case of recovering a polymerization inhibitor with water extraction, similar bad influences are brought because water-soluble impurities are collected simultaneously therewith.
Furthermore, the operations of the above (1) and (2) also had a problem of difficulty in stable continuous operation because a solution to be treated, which contains the polymerization inhibitor, has high viscosity, and in the operations, a lowering an ability of a device owing to its dirt and a blockage tend to occur.